Effect of physical aging of poly(1‐trimethylsilyl‐1‐propyne) films synthesized with TaCl5 and NbCl5 on gas permeability,fractional free volume,and positron annihilation lifetime spectroscopy parameters

The effect of physical aging on the gas permeability, fractional free volume (FFV), and positron annihilation lifetime spectroscopy (PALS) parameters of dense, isotropic poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) films synthesized with TaCl₅ and NbCl₅ was characterized. As‐cast films were soaked in methanol until an equilibrium amount of methanol was absorbed by the polymer. When the films were removed from methanol, film thickness initially decreased rapidly and was almost constant after 70 h in air for both catalysts. This timescale was much longer than the timescale for complete methanol desorption (ca. 5 h). From the film‐thickness data, the reduction in FFV with time was estimated. For samples prepared with either catalyst, the kinetics of FFV reduction were well‐described by a simple model based on the notion either that free‐volume elements diffuse to the surface of the polymer film and are subsequently eliminated from the sample or that lattice contraction controls polymer densification. Methane permeability decreased rapidly during the first 70 h, which was the same timescale for the thickness change. The decrease in methane permeability was smaller in films prepared with NbCl₅ than with TaCl₅. The logarithm of methane permeability decreased linearly as reciprocal FFV increased, in accordance with free‐volume theory. The PALS results indicate that the concentration of larger free‐volume elements (as indicated by the intensity I₄) decreased with aging time and that the other PALS parameters were not strongly influenced by aging. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1222–1239, 2000     

Crosslinking and stabilization of nanoparticle filled PMP nanocomposite membranes for gas separations

Poly(4-methyl-2-pentyne) (PMP) has been crosslinked using 4,4′-(hexafluoroisopropylidene) diphenyl azide (HFBAA) to improve its chemical and physical stability over time. Crosslinking PMP renders it insoluble in good solvents for the uncrosslinked polymer. Gas permeability and fractional free volume (FFV) decreased as crosslinker content increased, while gas sorption was unaffected by crosslinking. Therefore, the reduction in permeability upon crosslinking PMP was due to decrease in diffusion coefficient. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the crosslinking. By adding nanoparticles (FS, TiO₂), the permeability is again increased; permeability reductions due to crosslinking could be offset by adding nanoparticles to the membranes. Increased selectivity is documented for the gas pairs O₂/N₂, H₂/N₂, CO₂/N₂, CO₂/CH₄ and H₂/CH₄ using crosslinking and addition of nanoparticles. Crosslinking is successful in maintaining the permeability and selectivity of PMP membranes and PMP/filler nanocomposites over time.     

Cross-linking of poly[1-(trimethylsilyl)-1-propyne] membranes using bis(aryl azides)

Cross-linkable poly[1-(trimethylsilyl)-1-propyne] (PTMSP) films were cast from toluene solutions containing PTMSP and either 4,4′-diazidobenzophenone or 4,4′-(hexafluoroisopropylidene)diphenyl azide. The composite films were clear and homogeneous and were cross-linked by UV irradiation at room temperature or thermal annealing at 180°C. Low levels of the bis(aryl azide) (1–5 wt %) were effective in rendering the films insoluble in toluene and THF, both good solvents for PTMSP. The process is simple and effective, and thus PTMSP can be readily converted to mechanically stable membranes with permeabilities and separation factors comparable or higher than those of poly(dimethylsiloxane). The films were characterized by measuring their density, their permeability toward O₂ and N₂, and their spectroscopic properties. Compared to PTMSP, films containing bis(aryl azide) cross-linkers had lower permeabilities and higher separation factors, consistent with a reduction in free volume. When the films were cross-linked photochemically, the permeabilities declined further and the separation factor increased. Films cross-linked thermally had permeabilities comparable to their PTMSP/azide precursors, and density and swelling measurements suggest that higher free volumes are obtained in thermally cross-linked films. All films stored in air suffered from a slow decline in permeability which may reflect slow surface oxidation of the films. When stored in vacuum, cross-linked films were stable and showed no loss in permeability, but the permeability of uncross-linked PTMSP films stored under the same conditions fell to 70% of their original value in 1 month. We attribute the permeability decline to densification accelerated by impurities and solvents. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 959–968, 1998     

Pervaporation dehydration of ethylene glycol with chitosan–poly(vinyl alcohol) blend membranes: Effect of CS–PVA blending ratios

Chitosan–poly(vinyl alcohol), CS–PVA, blended membranes were prepared by solution casting of varying proportions of CS and PVA. The blend membranes were then crosslinked interfacially with trimesoyl chloride (TMC)/hexane. The physiochemical properties of the blend membranes were determined using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), tensile test and contact angle measurements. Results from ATR-FTIR show that TMC has crosslinked the blend membranes successfully, and results of XRD and DSC show a corresponding decrease in crystallinity and increase in melting point, respectively. The crosslinked CS–PVA blend membranes also show improved mechanical strength but lower flexibility in tensile testing as compared to uncrosslinked membranes. Contact angle results show that crosslinking has decreased the surface hydrophilicity of the blend membranes. The blend membrane properties, including contact angle, melting point and tensile strength, change with a variation in the blending ratio. They appear to reach a maximum when the CS content is at 75 wt%. In general, the crosslinked blend membranes show excellent stability during the pervaporation (PV) dehydration of ethylene glycol–water mixtures (10–90 wt% EG) at different temperatures (25–70 °C). At 70 °C, for 90 wt% EG in the feed mixture, the crosslinked blend membrane with 75 wt% CS shows the highest total flux of 0.46 kg/(m² h) and best selectivity of 986. The blending ratio of 75 wt% CS is recommended as the optimized ratio in the preparation of CS–PVA blend membranes for pervaporation dehydration of ethylene glycol.     

Crosslinking of hyaluronic acid with glutaraldehyde

Hyaluronic acid (HA) was chemically crosslinked with glutaraldehyde (GA) to produce water-insoluble films having low water contents when brought into contact with water. The crosslinking reaction was performed using uncrosslinked HA films in acetone–water mixtures. This method could produce water-insoluble HA films with water contents as low as 60 wt % when subjected to swelling with phosphate-buffered saline of pH 7.4 at 37°C. This 60 wt % water content was lower than any values for HA ever reported. There was an optimal HCl concentration around 0.01N for the HA crosslinking with GA in acetone—water mixtures. To get information on the crosslinking mechanism, alginic acid, which possesses hydroxyl and carboxyl groups in one molecule, similar to HA, and poly(vinyl alcohol) (PVA) and amylopectin, which possess only hydroxyl groups, were subjected to crosslinking with GA. PVA and amylopectin were also found to become water-insoluble after reaction with GA. On the basis of the infrared spectra of these crosslinked films, it was concluded that intermolecular formation of hemiacetal bonds with GA between the hydroxyl groups belonging to different HA molecules led to crosslinking. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3553–3559, 1997     

Membranes for gas separation based on poly(1-trimethylsilyl-1-propyne)–silica nanocomposites

Nanocomposite membranes based on poly(1-trimethylsilyl-1-propyne) (PTMSP) and silica were synthesized by sol–gel copolymerization of tetraethoxysilane (TEOS) with different organoalkoxysilanes in tetrahydrofuran solutions of PTMSP. The influence of the synthesis parameters (type and concentration of organoalkoxysilanes, temperature and time) on the silica conversion and the gas permeation performance of PTMSP–silica nanocomposite membranes was investigated and discussed in this paper. The nanocomposite membranes were characterized by single and mixed gas permeation, thermogravimetric analysis and scanning electron microscopy. The butane permeability and the butane/methane selectivity increased simultaneously when high silica conversion was obtained and the size of particle was in the range 20–40 nm. For the sake of comparison, nanocomposite membranes based on PTMSP were also prepared by dispersing silica particles with different functional groups into the PTMSP casting solution. The addition of fillers to the polymer matrix can be performed up to a higher content of silica (30% silica-filled PTMSP in contrast to 6 wt.% for the in situ-generated silica). In this case, the simultaneous increase in butane permeability and butane/methane selectivity was significantly higher when compared to the nanocomposite membranes prepared by sol–gel process. The addition of fillers with 50% of surface modification with hydrophobic groups (Si–C₈H₁₇ and Si–C₁₆H₃₃) seems not to lead to a significant increase of the butane/methane selectivity and butane permeability when compared to the silica with hydrophilic surface groups, probably because of the unfavorable polymer/filler interaction, leading to an agglomeration of the long n-alkyl groups at the surface of the polymer. An increase of butane permeability up to six-fold of unfilled polymer was obtained.     

Extraction of butanol from aqueous solutions by pervaporation through poly(1-trimethylsilyl-1-propyne)

Poly(1-trimethylsilyl-1-propyne) (PTMSP) was synthesized using a TaCl₅–Al(i-Bu)₃ catalysis system. Pervaporation and sorption of n-butanol–water mixtures were studied, and the peculiarities of water and butanol co-permeation are discussed. The strong dependence of water partial flux (with a minimum at 1 wt.% butanol in feed) on butanol concentration in feed was observed. S-shaped isotherms of butanol and total sorption were found for PTMSP in 0–1 wt.% concentration range. It appears that blocking of PTMSP nanopores by high sorbing organic molecules controls the pervaporation of butanol from dilute aqueous solutions. Data are discussed in regard with PTMSP morphology.     

Cross-linking of Polymer of Intrinsic Microporosity (PIM-1) via nitrene reaction and its effect on gas transport property

Polymer of Intrinsic Microporosity (i.e. PIM-1) has been crosslinked thermally via nitrene reaction using polyethylene glycol biazide (PEG-biazide) as a crosslinker. The crosslinking temperature was optimized using TGA coupled with FT-IR spectroscopy. The dense membranes containing different ratios of PIM-1 to PEG-biazide were cast from chloroform solution. Crosslinking of PIM-1 renders it insoluble even in excellent solvents for the uncrosslinked polymer. The resulting crosslinked membranes were characterized by FT-IR spectroscopy, TGA and gel content analysis. The influence of crosslinker content on the gas transport properties of PIM-1, its density and fractional free volume (FFV) were investigated. Compared to the pure PIM-1 membrane, the crosslinked PIM-1 membranes showed better gas separation performance especially for CO₂/N₂, CO₂/CH₄ and propylene/propane (C₃H₆/C₃H₈) gas pairs and as well as suppressed penetrant-induced plasticization under high CO₂ pressure.     

Cathodic electrophoretic deposition of manganese dioxide films

Cathodic electrophoretic deposition (EPD) method has been developed for the deposition of manganese dioxide films. It was shown that phosphate ester (PE) is an effective charging additive, which provides stabilization of manganese dioxide nanoparticles in suspensions. The influence of PE concentration and deposition voltage on the deposition efficiency has been studied. EPD has been utilized for the fabrication of porous nanostructured films with thickness in the range of 0.5–20 μm for application in electrochemical supercapacitors (ES). Cyclic voltammetry and chronopotentiometry data for the films tested in the 0.1 M Na₂SO₄ solutions showed capacitive behavior in the voltage window of 1 V. The highest specific capacitance (SC) of 377 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹. The SC decreased with increasing film thickness and increasing scan rate in the range of 2–100 mV s⁻¹. The deposition mechanism, kinetics of deposition and charge storage properties of the films are discussed.     

XPS STUDIES ON SURFACE MODIFIED POLY [1-(TRIMETHYLSILYL)-1-PROPYNE] MEMBRANES Ⅱ SURFACE MODIFICATION BY BROMINE VAPOR*

Surface modification of poly [1-(trimethylsilyl)-1-propyne] (PTMSP) membranes bybromine vapor has been studied. It is shown that Br/C atomic ratio at the surfaces increased withthe time of bromination until about 60 min, then it reached a plateau. The results of XPS and IRstudies indicated that the addition of bromine to double bonds and the replacement of H on CH_3 bybromine had taken place so that a new peak at 286.0 eV (C--Br)in C_(1s) spectra and some newbands, e. g. at 1220 and 580cm~(-1) in IR spectra were formed. The fact,t Po_2, permeability ofoxygen, decreased and α_(O_2/N_2), separation factor of oxygen relative to nitrogen, increased withbromination time, shows that surface modification of PTMSP by bromine may be an efficient approach to prepare PTMSP membranes used for practical gas separations.     

Poly(4‐vinylpyridine) Nanocrosslinked by Polyhedral Oligomeric Silsesquioxane

Summary: Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) was used as the crosslinking agent to prepare the nanocrosslinked poly(4‐vinylpyridine) (P4VP) with POSS content up to 55.2 wt.‐%. The formation of the crosslinked structure is ascribed to the macromolecular reaction between pyridine rings of P4VP and epoxide groups of OpePOSS. The POSS‐crosslinked P4VP displayed enhanced glass transition temperatures (T_gs) and an improved thermal stability in terms of the results of thermal analysis.

Crosslinking of poly(4‐vinylpyridine) with octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane.     

Network properties of mixtures of protonated and deuterated polyethylene

Mixtures of protonated and deuterated polyethylene were irradiated in the melt. The degree of crystallinity, the degree of crosslinking, as well as the enthalpyH and the melting pointT _M were determined. No significant differences in the degree of crosslinking between protonated and deuterated chains were found. The mass specific entropyS of the uncrosslinked samples remained constant and independent of the deuterium concentration. For the crosslinked samples, a netpoint entropy was postulated. A weaker Van der Waals interaction could explain the decrease in melting temperature by deuteration (for weakly crosslinked samples).     

Unique size-change behavior of photo-crosslinked cinnamic acid derivative nanoparticles during hydrolytic degradation

A unique size change of photo-crosslinkable poly[(3,4-dihydroxycinnamic acid)-co-(4-hydroxycinnamic acid)] nanoparticles was observed during hydrolytic degradation depending on the crosslinking degree. The diameter of uncrosslinked nanoparticles decreased from 850 to 300 nm during hydrolysis, whereas that of 75% crosslinked nanoparticles increased from 700 to 950 nm. The diameter changes of crosslinked nanoparticles during hydrolysis might be induced by swelling of the crosslinked networks depending on the crosslinking degree. Moreover, the diameter of the uncrosslinked nanoparticle recovered by additional UV irradiation during hydrolysis. These results suggested that the diameter of the nanoparticles could be controlled even during hydrolysis by UV irradiation.     

Molecular simulations of physical aging in polymer membrane materials

Poly(1-trimethylsilyl-1-propyne) (PTMSP), the most permeable polymer known, undergoes rapid physical aging. The permeability of PTMSP to gases and vapors decreases dramatically with physical aging. Cavity size (free volume) distributions were calculated in as-cast and aged PTMSP, using an energetic based cavity-sizing algorithm. The large cavities found in as-cast PTMSP disappear in aged PTMSP, which is consistent with the positron annihilation lifetime spectroscopy (PALS) measurements. We also characterized the connectivity of cavities in both as-cast and aged PTMSP membranes. Cavities are more connected in as-cast PTMSP than in aged PTMSP. The average cavity sizes calculated from computer simulation are in good agreement with PALS measurements. The transport and sorption properties of gases in as-cast and aged PTMSP are also measured by molecular simulation. Computer simulations showed the decrease of permeability and the increase of permeability selectivity in PTMSP membranes with physical aging, which agrees with experimental observations. The reduction in gas permeability with physical aging results mainly from the decrease of diffusion coefficients. Solubility coefficients show no significant changes with physical aging.     

老化对聚(1-三甲基硅基-1-丙炔)气体渗透性能的影响

采用溶液浇铸法,制备了厚度为50~202 μm的聚(1-三甲基硅基-1-丙炔)(PTMSP)膜,研究了膜厚度、储存温度以及储存气氛对其气体渗透性能的影响。 在室温下储存时,PTMSP膜发生物理老化,气体渗透系数先是迅速下降,然后缓慢降低并趋向平稳。 在空气气氛中的下降速率要略大于在N2气气氛中。 气体渗透系数的下降速率随膜厚度的减小而增大。 在高温(100 ℃)空气气氛中,受物理及化学老化的共同作用,PTMSP膜气体渗透系数的下降速率进一步增大,IR谱图表明,聚合物氧化生成了C=O等极性基团。 随储存时间的延长,溶解度系数基本不变,扩散系数的下降是导致气体渗透系数下降的主要原因,这与聚合物体积松弛和(或)致密化及极性基团的形成所造成的自由体积的减小紧密相关。     

Preparation of aqueous crosslinked dispersions of functionalized poly(d,l-lactic acid) with a thermomechanical method

Aqueous crosslinked microparticle dispersions were prepared from a copolymer of d,l-lactic acid, 1,4-butanediol, and itaconic acid with a thermomechanical method. The copolymer was prepared in one step polycondensation reaction using Sn(Oct)₂ as a catalyst. A polymer with M_n of 2800 g mol^?1 and a molecular weight distribution of 1.41 was obtained (as determined by SEC), that contained double bonds introduced by the itaconic acid monomer units (6 mol-%, as determined by NMR). Crosslinking ability of the prepared copolymer was demonstrated in bulk by adding a thermal initiator and altering amounts of ethylene glycol dimethacrylate (EGDMA) crosslinking agent into molten polymer at 60–150 °C. A crosslinked gel was formed in less than 15 min at 80 °C when 10 wt.% of EGDMA was added and benzoyl peroxide (BPO) was used as the initiator. Aqueous dispersions were prepared of the non-crosslinked copolymer with a thermomechanical method that involved slow addition of aqueous polyvinyl alcohol (PVA) solution into molten copolymer at 60 °C under shear. Dispersions were prepared with 10 wt.% of EGDMA and 2 wt.% of BPO. Crosslinking of the dispersed microparticles was achieved by heating the dispersions at 80 °C for 30 or 60 min. The dispersions were characterized by SEM, DSC, TGA, FT-IR, solid state NMR, and gel content measurements. The effect of crosslinking was clearly seen in SEM images of films cast from the dispersions. The films cast from non-crosslinked dispersions had smooth morphology whereas in films cast from crosslinked dispersions separate spherical particles were observed. During the crosslinking reactions, glass transition temperatures increased (as determined by DSC), thermal stability of the samples increased (as determined by TGA), and the gel content of the samples increased.     

Preparation and characterization of stable monodisperse silver nanoparticles via photoreduction

Silver nanoparticles were synthesized by UV irradiation of [Ag(NH₃)₂]⁺ aqueous solution using poly(N-vinyl-2-pyrrolidone) (PVP) as both reducing and stabilizing agents. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption maxima around 420 nm. It was found that the formation rate of silver nanoparticles from Ag₂O was much quicker than that from AgNO₃, and the absorption intensity increased with PVP concentration as well as irradiation time. The maximum absorption wavelength (λ_max) was blue shift with increasing PVP content until 8 times concentration of [Ag(NH₃)₂]⁺ (wt%). The transmission electron microscopy (TEM) showed the resultant particles were 4–6 nm in size, monodisperse and uniform particle size distribution. X-ray diffraction (XRD) demonstrated that the colloidal nanoparticles were the pure silver. In addition, the silver nanoparticles prepared by the method were stable in aqueous solution over a period of 6 months at room temperature (25 °C).     

Effect of crosslinking on the properties of composites based on LDPE and conducting organic filler

New types of composites were prepared using low-density polyethylene (LDPE) filled with modified organic filler, Canadian switch grass coated with polypyrrole (PPy). The grass surface was entirely covered when 10 wt.% of pyrrole was used for the modification, as confirmed by scanning electron microscopy and infrared spectroscopy. LDPE composites filled with modified grass were prepared by melt mixing and their properties were compared with the properties of the composites filled with unmodified grass. The influence of crosslinking, induced by 1 wt.% of peroxide, on mechanical, thermal and electrical properties of the composites was investigated. Crosslinking enhanced the tensile strength of the prepared composites in the entire range of the filler content. The Young’s modulus of the composites prepared by crosslinking is slightly lowered when compared with the uncrosslinked composites if the filler content is less than 60 wt.%, for higher filler content it is increased. The conductivity of the uncrosslinked composites containing 40 wt.% of grass modified by PPy was in the range 1 × 10⁻⁶ S cm⁻¹, which is a value by 5 orders of magnitude higher than the conductivity of the crosslinked materials. The presence of PPy on grass surface leads to a reduction of crosslinking of the LDPE matrix.     

Electroreduction of anions on chemically etched and electrochemically polished Bi(1 1 1) electrode

Electroreduction kinetics of to anions at chemically etched (CHE) and electrochemically polished (EP) Bi(1 1 1) electrodes has been studied using rotating disc electrode method. The surface nanostructure of CHE Bi(1 1 1) and EP Bi(1 1 1) electrodes has been studied by in situ STM and the very different values of root mean squared roughness (Rms) have been obtained (1000 times higher for CHE Bi(1 1 1) (Rms  143 nm) than for EP Bi(1 1 1) (Rms  0.145 nm)). The influence of the nanoroughness of CHE Bi(1 1 1) on the current density, heterogeneous reaction rate constant and corrected Tafel plots (cTp) has been demonstrated. For CHE Bi(1 1 1) the more pronounced inhibition of electroreduction reaction at moderate negative surface charge density has been observed in comparison with EP Bi(1 1 1), caused by the differences in surface charge density and also in diffuse layer ψ₀ potential drop values at crystallographically different homogeneous regions (planes) exposed at the surface of the macroheterogeneous polycrystalline CHE Bi(1 1 1) surface. The very low apparent transfer coefficient α_app obtained indicates the nearly activationless charge transfer mechanism for electroreduction at the CHE Bi(1 1 1) electrode similarly to EP Bi(1 1 1). However, α_app only very weakly depends on Rms for the Bi electrodes at high negative surface charge densities where the values of ψ₀ potential are nearly equal for different planes at fixed electrode potential. At very high negative surface charge densities the cationic catalysis through the adsorbed ion pairs is possible.     

Sorption,diffusion, and permeation of ethylbenzene in poly(1‐trimethylsilyl‐1‐propyne)

The solubility, diffusivity, and permeability of ethylbenzene in poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) at 35, 45 and 55 °C were determined using kinetic gravimetric sorption and pure gas permeation methods. Ethylbenzene solubility in PTMSP was well described by the generalized dual‐mode model with χ = 0.39 ± 0.02, b = 15 ± 1, and C^′_H = 45 ± 4 cm³ (STP)/cm³ PTMSP at 35 °C. Ethylbenzene solubility increased with decreasing temperature; the enthalpy of sorption at infinite dilution was −40 ± 7 kJ/mol and was essentially equal to the enthalpy change upon condensation of pure ethylbenzene. The diffusion coefficient of ethylbenzene in PTMSP decreased with increasing concentration and decreasing temperature. Activation energies of diffusion were very low at infinite dilution and increased with increasing concentration to a maximum value of 50 ± 10 kJ/mol at the highest concentration explored. PTMSP permeability to ethylbenzene decreased with increasing concentration. The permeability estimated from solubility and diffusivity data obtained by kinetic gravimetric sorption was in good agreement with permeability determined from direct permeation experiments. Permeability after exposure to a high ethylbenzene partial pressure was significantly higher than that observed before the sample was exposed to a higher partial pressure of ethylbenzene. Nitrogen permeability coefficients were also determined from pure gas experiments. Nitrogen and ethylbenzene permeability coefficients increased with decreasing temperature, and infinite dilution activation energies of permeation for N₂ and ethylbenzene were −5.5 ± 0.5 kJ/mol and −74 ± 11 kJ/mol, respectively. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1078–1089, 2000